1. Field of the Invention
The present invention relates generally to charged particle beam apparatus, and more specifically to electron microscopy and electron beam patterning methods.
2. Description of the Background Art
Optical microscopes, the simplest and most used instruments used to image objects too small for the naked eye to see, uses photons with visible wavelengths for imaging. The specimen is illuminated with a broad light beam, and a magnified image of the specimen can be observed using an eye piece or camera. The maximum magnification of a light microscope can be more than 1000× with a diffraction-limited resolution limit of a few hundred nanometers. Improved spatial resolution in an optical microscope can be achieved when shorter wavelengths of light, such as the ultraviolet, are utilized for imaging.
An electron microscope is a type of microscope that uses electrons to illuminate the specimen and create a magnified image of it. The microscope has a greater resolving power than a light microscope, because it uses electrons that have wavelengths few orders of magnitude shorter than visible light, and can achieve magnifications exceeding 1,000,000×. In a typical electron microscope, an electron beam is emitted in a vacuum chamber from an electron gun equipped with a thermionic (tungsten, LaB6), thermally assisted (Schottky, ZrO2) or cold field emission cathode. The electron beam, which typically has an energy ranging from a few hundred eV to few hundred keV and an energy spread ranging from few tenths to few eV, is collimated by one or more condenser lenses and then focused by the final objective lens to form a spot that illuminates the specimen. When the primary electron beam strikes the sample, the electrons deposit energy in a teardrop-shaped volume of the specimen known as the interaction volume, which extends from less than few nm to few μm into the surface, depending on the electron's landing energy and the composition of the specimen. Primary electrons can generate elastically scattered electrons, secondary electrons due to inelastic scattering, characteristic Auger electrons and the emission of electromagnetic radiation. Each of the generated signals can be detected by specialized detectors, amplified and displayed on a CRT display or captured digitally, pixel by pixel on a computer.
Scanning electron microscopes, the most widely used electron microscopes, image the sample surface by scanning it with a tightly focused beam of electrons in a raster scan pattern, pixel by pixel. Transmission electron microscopes (TEM) and low energy electron microscopes (LEEM) are projection (as opposed to scanning) electron microscopes, and thus resemble a conventional light microscope. In a TEM or LEEM, the electron gun forms a broad electron beam that is accelerated to typically a few to hundreds of keV and focused by the objective lens. A parallel flood beam then uniformly illuminates the substrate.
The finite, non-zero energy spread ΔE of the illuminating energy beam introduces chromatic aberrations that deteriorate the spatial resolution of electron beam instruments, including both scanning and projection electron microscopes as well as electron beam pattern generators. The primary chromatic aberration is proportional to the relative energy spread ΔE/E, where E is the nominal beam energy. Since the chromatic aberration increases with decreasing beam energy, an appreciable improvement of the resolution can be achieved when the energy spread ΔE is reduced, in particular at low beam energies. The energy spread of commonly used thermionic (tungsten, LaB6) and thermally assisted (Schottky, ZrO2) field emission cathodes is typically in the range of 0.5 to 5 eV, and cold field emitters have an energy spread in the range of 0.3-0.5 eV. Effective means for reducing the energy spread of the primary electron beam illuminating the sample to 0.1 eV or less are therefore desirable for higher spatial resolution imaging and patterning.
One approach to reducing the energy spread of the primary electron beam illuminating the sample is to use an monochromator based on a Wien-type energy filter, such as the one disclosed in U.S. Pat. No. 5,838,004, which is entitled “Particle-optical apparatus comprising a fixed diaphragm for the monochromator filter” and which issued Nov. 17, 1998 to inventors Tiemeijer, Chmelik and Kruit. In this approach, the monochromator is located in the vicinity of the electron source and at high electric potential, where the kinetic energy of electrons is low and the Wien filter most effective. However, the energy dispersion of the Wien filter is rather low, and extremely narrow energy-selecting slits, 0.1 micrometer wide or smaller, must be employed. The manufacture of such fine structures is rather complicated and the reliability of operation under heavy electron bombardment is reduced due to hydrocarbon contamination. In addition, the design of the Wien filter and its electronic components is complicated due to the fact that the components are floating at high electrical potential.
Another approach to reducing the energy spread of the primary electron beam illuminating the sample is to use an omega-type energy filter, such as the one disclosed in U.S. Pat. No. 5,126,565, which is entitled “Energy filter for charged particle beam apparatus” and which issued Jun. 30, 1992 to inventor Rose. In this approach, the monochromator is located in the vicinity of the electron source and comprises 4 symmetrically arranged sector deflectors which introduce a dispersion which has a maximum at the center of the filter. However, the energy dispersion of the omega-type energy filter is still low. This means that when a energy width in the range of 0.1 eV is required, the filter must also be biased at high electric potentials, and narrow energy-selecting slits, about 1 micrometer wide, must be employed. The manufacture of such fine structures with straight and parallel edges is rather complicated. In addition, the design of the omega filter and its electronic components is complicated due to the fact that the components are floating at high electrical potential.
There is significant demand in biological and medical research as well materials science and semiconductor processing for imaging of specimens at high spatial resolution and with analytical capabilities provided by scanning and projection electron microscopes equipped with monochromators, as well as patterning of substrates at high spatial resolution provided by electron beam pattern generators equipped with monochromators.